1. Field of the Invention
The present invention relates generally to waveguide to microstrip conversion means and more specifically, to a waveguide to microstrip conversion means in a satellite broadcasting antenna for receiving first and second linearly polarized waves orthogonal to each other.
2. Description of the Background Art
Conventional outdoor converters for receiving satellite broadcasting have been known which are installed on outdoor antennas for receiving two kinds of independent linearly polarized waves (horizontally polarized wave and vertically polarized wave) orthogonal to each other.
FIG. 20 is a partial perspective view showing the internal structure of a conventional waveguide to microstrip converter used for a conventional outdoor converter for receiving satellite broadcasting. Referring to FIG. 20, the converter includes a circular waveguide 101, a first microstrip circuit substrate 102 attached at a prescribed position in the outer periphery of the circular waveguide, a first probe 105 attached on first microstrip circuit substrate 102 and formed protruding as far as the interior of circular waveguide 101 through its opening, a short circuit element (shorting rod) 104 provided inside circular waveguide 101, being 1/4 wavelength spaced apart and downwardly from the position where first probe 105 is attached, a second probe 106 formed protruding into circular waveguide 101 and about a half wavelength spaced apart downwardly from first probe 105, a second microstrip circuit substrate 103 provided with second probe 106 through the opening of circular waveguide 101 and attached to the outer periphery of circular waveguide 101, and a short circuit element (shorting plate) 107 attached a 1/4 wavelength apart downwardly from second probe 106.
First probe 105 and short circuit element 104 are disposed so that their extending directions are approximately the same. First probe 105 and second probe 106 are disposed so that their extending directions are orthogonal to each other. First probe 105 is for detecting a horizontally polarized wave 1, and second probe 106 is for detecting a vertically polarized wave 2. Short circuit terminal (shorting rod) 104 reflects the horizontally polarized wave 1 and allows its detection by first probe 105, and short circuit terminal (shorting plate) 107 reflects the vertically polarized wave 2 and allow its detection by second probe 106.
Two probes are generally necessary for a single waveguide to receive two kinds of linearly polarized waves orthogonal to each other. Each of the probes receives a signal having a plane of polarization parallel to them. In order to prevent interference of signals having a plane of polarization other than the plane parallel to the probe and improve cross polar discrimination, as shown in FIG. 20, it will be necessary to dispose the first probe 105 and second probe 106 of circular waveguide 101 so as to be orthogonal to each other and coaxially spaced apart from each other. In order to improve the input VSWR (Voltage Standing Wave Ratio) of the waveguide and to prevent conversion losses at first probe 105 and second probe 106 within a broad band width, first probe 105 and shorting rod 104 as well as second probe 106 and shorting plate 107 should be both spaced apart from each other by 1/4 wavelength.
FIG. 21 is a perspective view showing the internal structure of an alternative type of a conventional waveguide to microstrip converter. Referring to FIG. 21, the converter includes a circular waveguide 201, a microstrip circuit substrate 202 attached at a predetermined position in the outer periphery of circular waveguide 201, a first probe 203 attached on microstrip circuit substrate 202 and protruding into circular waveguide 201, a second probe 204 attached to microstrip circuit substrate 202 and protruding into circular waveguide 201 in the direction orthogonal to first probe 203 on the same plane, and a short circuit terminal (shorting plate) 205 disposed 1/4 wavelength spaced apart downwardly from first probe 203 and second probe 204. In this conventional waveguide to microstrip converter, the first probe 203 and second probe 204 are positioned on the same plane. Thus providing first probe 203 and second probe 204 on the same plane reduces the size of the apparatus.
As in the foregoing, in the structure of the conventional waveguide to microstrip converter shown in FIG. 20, first probe 105 and second probe 106 are spaced about half wavelength apart from each other, in order to prevent interference of a signal received by first probe 105 and a signal received by second probe 106 thereby improving cross polar discrimination.
However in such a structure, with first probe 105 not being coplanar with second probe 106, it is not possible to directly connect both first probe 105 and second probe 106 to a single microstrip circuit substrate. As shown in FIG. 20, two microstrip circuit substrates, first microstrip circuit substrate 102 and second microstrip circuit substrate 103 are necessary as a result. This increases the number of parts and complicates the manufacturing process, thus impeding cost reduction and productivity improvements. Additionally, with the separation distance between first probe 105 and second probe 106 being substantial, circular waveguide 101 results in a relatively long structure. This makes it difficult to reduce the size and weight of the apparatus.
In the case of the other conventional waveguide to microstrip converter shown in FIG. 21, as in the foregoing, the provision of first probe 203 and second probe 204 on the same plane allows for reduction of the size of the apparatus. In the structure shown in FIG. 21, however, as first probe 203 and second probe 204 are on the same plane, they both suffer from the interference of signals having a plane of polarization other than the planes of polarization they are supposed to receive. Consequently, the apparatus suffers from the disadvantage that the cross polar discrimination is degraded.
As described above, conventionally the structure of the apparatus should be expanded at the sacrifice of size reduction in order to provide good cross polar discrimination and input VSWR. If first probe 203 and second probe 204 are formed on the same plane for reducing the size of the apparatus, cross polar discrimination deteriorates.